Electrostatic spraying apparatus

ABSTRACT

An apparatus is disclosed suitable for electrostatic spraying from fixed wing aircraft. The apparatus includes a linear electrostatic spraying nozzle 2 and electrodes 4 placed near the nozzle&#39;s spraying edge to intensify the electric field strength at the spraying edge sufficiently to produce ligaments of the liquid to be sprayed from the spraying edge. In order that the airstream due to the aircraft&#39;s movement does not destroy the ligamets, the sprayhead and the electrodes are positioned so that part of the airstream flows between them. The spray head and the electrodes are so shaped and positioned that when directed to spray in substantially the same direction as the airstream, a turbulence free wake is left in the region of the ligaments.

FIELD OF THE INVENTION

This invention relates to electrostatic spraying apparatus.

BACKGROUND OF THE INVENTION

One of the advantages of electrostatic spraying is that the chargedspray tends to wrap around the target. This can be of particular use in,say, agricultural spraying because the spray will cover both sides ofthe leaves of a plant, not merely the outer or upper surfaces as wouldbe achieved with conventional spraying. Another feature is that theattraction of the spray to the target may reduce the amount lost bydrift. More or most of the spray reaches its intended target. Thisreduces the total amount of spray which has to be used which reduces thecost of effective treatment and is thought to be generally better forthe environment.

Electrostatic spraying apparatus is known in which a spray head has aspraying edge, an electrically conducting or semiconducting surface andmeans for delivering liquid to be sprayed to the edge via the surface;an electrode spaced from the edge; and high voltage supply means forgenerating a high voltage between the surface and the electrode so that,in use, when covered by the liquid to be sprayed, the electric fieldstrength at the edge is intensified sufficiently, that the liquid at theedge is drawn out preponderantly by electrostatic forces into ligamentswhich break up into electrically charged droplets.

An apparatus falling within this broad type is disclosed in Britishpatent specification No. 1569707.

An advantage of this apparatus is that the ligaments break up intodroplets having a very narrow spectrum of diameters. This is preferredbecause if a droplet of a particular size is required to carry a lethaldose of an insecticide, say, smaller droplets are wasted as ineffectivewhile larger droplets require a larger amount of insecticide to providethe same number of sites.

In order to treat large areas spraying can be effected from aircraft.Although aerial electrostatic spraying has been proposed, e.g. Europeanpatent application No. EP-A1-186353, a problem which has not beenaddressed is that caused by the airstream past the aircraft. In fixedwing aircraft used for spraying, there is an airstream past the vehicledue to its movement and possibly accentuated by the slipstream from apropeller, of the order of 70 mph. The problem cause by the airstream,is that turbulence around the electrostatic spray head interferes withthe formation of the ligaments and thus spoils the spectrum of dropletdiameters or even prevents spraying.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided apparatus forspraying liquid electrostatically into an airstream comprising: a sprayhead having a spraying edge, an electrically conducting orsemiconducting surface and means for delivering liquid to be sprayed tothe edge via the surface; an electrode spaced from the edge; and highvoltage supply means for generating a high voltage between the surfaceand the electrode, the sprayhead comprising an aerofoil the trailingedge of which constitutes the spraying edge, the sprayhead and theelectrode being mounted for part of the airstream to pass between them,the shape and position of the sprayed and the electrode producing asufficiently low turbulence wake in the region of the spraying edge, andthe electric field at the edge being intensified sufficiently whencovered by liquid to be sprayed, that: the liquid at the edge is drawnout preponderantly by electrostatic forces into ligaments which break upinto elactrically charged droplets.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the invention will now be described by way of example,with reference to the accompanying drawings, in which:

FIGS. 1 and 2 show the general disposition on a light aircraft ofapparatus embodying the invention;

FIG. 3 shows a cross section through spraying apparatus embodying theinvention;

FIGS. 3a and 3b show details of FIG. 3;

FIG. 4 shows an end view of an alternative embodiment;

FIG. 5 shows the fluid delivery arrangement of the embodiment of FIG. 3;and

FIG. 6 shows the electrical circuit of the embodiment of FIG. 3.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, a linear sprayhead 2a, 2b is mounted beneaththe trailing edges of each wing of a light aircraft 3. The position ofthe sprayhead is chosen so that the sprayhead is in turbulence free airand such that the spray is directed substantially parallel to the airflow and does not end up in any substantial quantity on the tail planeof the aircraft. The sprayhead is supported by arms 6 (FIG. 1) attachedto brackets 8 (FIG. 3) at intervals of about one half meter.

The spray head 2, shown in FIG. 3, is in the form of an aerofoil body(which in the figure is symmetrical) at the trailing edge of which is alinear nozzle. The body comprises a nose assembly 12 formed generally ofinsulating materials and a nozzle assembly 14 formed in this case of asemi insulating material, e.g. composite sold under the trade mark KiteBrand by Tufnol Limited of Birmingham England. The nozzle assembly 14provides the trailing edge 16 of the aeorfoil. The trailing edge 16 alsoacts as a spraying edge. The nozzle assembly 14 comprises two parts 14aand 14b secured together with a thin spacer therebetween leaving a slot18, defined by the thickness of the spacer, just forward of thetrailing/spraying edge 16.

In use a liquid agrochemical is delivered through the slot 18, via aconducting or semi conducting surface 20, across the exterior surface21, to the spraying edge 16 from which spraying takes place. Thespraying edge is directed between the two opposed electrodes 4.

The electrodes 4 comprise a core 22 of conducting material sheathed by abody 24 partly of semi-insulating material 26 and partly of insulatingmaterial 28. The insulating part 28 of the body 24 is formed from glassreinforced plastics by pultrusion. The semi insulating part of the body24 is a round tube 25 of a material having a resistivity in the range10¹⁰ to 10¹⁴, more preferably 5×10¹¹ to 5×10¹³ ohm cms. Examples ofsuitable materials are certain grades of soda glass andphenolformaldehyde/paper composites. The composite sold under the trademark Kite Brand by Tufnol Limited of Birmingham England has been foundparticularly suitable. The core 22 is packed iron filings or carbongranules. The tube 25 is bonded and faired to the insulating part 28, byan epoxy filler or adhesive 23.

The conducting or semiconducting surface 20 is connected via one of apair of supply leads (not illustrated) to one of the output terminals ofa high voltage generator 50 or 52 (FIG. 6). The electrode cores 22 areconnected by the other of the pair of high voltage supply leads toanother voltage output terminal of the high voltage generator, so thatin use a high potential difference, e.g. 10 to 35 Kv, is maintainedbetween the surface 20 and the electrode cores 22. Various voltageconfigurations can be used. Assuming the target is essentially at earthpotential, either the electrodes 22 or (as will be explained later) thesurface 20 may be at earth potential. Alternatively, the electrodes 22may be maintained at a potential intermediate that of the surface 20 andthat of the target. In our preferred arrangement, the surface 20 ismaintained at ±35 Kv and the electrodes are maintained at anintermediate potential of ±17.5 Kv. The electrodes thus have a potentialof similar polarity to that of the droplets in the spray. Once past theelectrodes the droplets thus tend to be repelled by the electrodes. Ifthe electrodes are at ground potential, there is a tendency, especiallyat high flow rates, for the droplets to be attracted back to theelectrodes.

Any suitable circuit arrangement may be used to provide the voltagesrequired at the surface 20 and the electrode cores. In FIG. 6 eachgenerator is illustrated with two high voltage outputs. In anotheralternative the electrode core voltage is derived by a potential dividerfrom a single output generator.

The edge 16 is sharp to a degree, that combined with the closeness withwhich the electrode cores 22 are spaced therefrom, enables spraying totake place at a relatively low high-voltage. In use the electric fieldis defined between the semi insulating part 26 of the electrode sheathsand the liquid arriving at the edge 16. Assuming the surface 20 has apositive potential relative to the electrode cores 22, negative chargeis conducted away from the liquid at its contact with the conducting orsemiconducting surface, leaving a net positive charge on the liquid. Thepresence of the electrodes 4 intensifies the electric field at theliquid/air boundary at the edge 16, sufficiently that the liquid isdrawn out into ligaments spaced along the edge 16.

The liquid becomes positively charged, negative charge being conductedaway by the conducting or semiconducting surface 20, leaving a netpositive charge on the liquid. The charge on the liquid producesinternal repulsive electrostatic forces which overcome the surfacetension of the liquid forming cones of liquid at spaced intervals alongthe edge 16. From the tip of each cone a ligament issues. At a distancefrom the edge 16, mechanical forces on the ligament produced bytravelling through the air cause it to break up into charged droplets ofclosely similar size. Mutual repulsion between the droplets causes thespray to expand in a direction transverse to the ligaments. The numberof ligaments, which are formed depends on the flow rate of the liquidand on the electric field intensity amongst other factors such as theresistivity and the viscosity of the liquid. All other things beingconstant, controlling the voltage and the flow rate controls the numberof ligaments, which enables the droplet size to be controlled and veryclosely similar.

If the conducting surface is separate from the spraying edge 16, we findit necessary to dimension the spacing therebetween suitably, in relationto the resistivity of the liquid being sprayed. We find that sprayingwill not take place if, given a spacing, the resistivity of the liquidis too high or, conversely, given a particular resistivity, the spacingis too great. A possible explanation for this observation is that inaddition to the liquid becoming charged as it passes over the conductingor semiconducting surface, there is also conduction of charge away fromthe liquid at edge 16 through the liquid. The resistance of this pathmust not be so high that the voltage drop across it results in thevoltage at the edge 16 being too low to produce an atomising fieldstrength. The distance between the edge 16 and the conducting orsemiconducting surface must therefore be sufficiently small to allow forthe resistivity of the liquid being used. We have found that a suitableposition can be found for the surface even when spraying, say, a liquidhaving a resistivity in the range 10⁶ to 10¹⁰ ohm cm.

Since the electrical connections are made to the core 22, the surface ofthe body 24 is not at a uniform potential. The surface potential will belowest on the semi insulating material 26 near the core 22 and it ishere that the flux in the electric field between the edge 16 and theelectrodes will concentrate. In order to permit maximum electricalstress to be applied between the edge 16 and the electrode body in theregion of the core 22, without surface tracking leading to coronadischarge between more closely spaced points, the core and the sheathare so shaped and positioned as to be closest to the spray head at thespraying edge. In the example illustrated, the core is packed ironfillings or carbon granules.

It is important that the area near the spraying edge where the ligamentsare formed is substantially free from airflow transverse to theligaments. This would spoil or even prevent formation of the ligaments.To this end, the three aerofoils are so shaped as to each leave a lowturbulence wake, when the spray head is substantially aligned with thegeneral direction of the airstream. In the area downstream of thespraying edge 16, it is desired to give the droplets as littleopportunity as practical to deposit on the electrodes. To this end, theelectrode bodies curve away from each other towards their trailing edgescreating an expanding passage. The airstream passing through thispassage is thus decelerating, creating an environment in which it isdifficult to remove turbulence completely. However, sufficiently lowturbulence can be achieved to permit the formation of stable ligamentsby electrostatic forces and in practice the arrangement can have anangle of incidence of 10 of 15 degrees or so to the general direction ofthe air stream, before it stalls creating a turbulent wake. This enablesspraying to take place through the normal range of attitudes of theaircraft.

At high spraying rates and/or high potential differences between thesurface 20 and the electrode cores 22, there is a tendency for dropletsof the spray to contaminate the electrodes. This tendency will bereduced by the air flow over the edge 16 which will assist the dropletsaway from the electrodes faster than they can migrate transverse to theairflow.

In the arrangement illustrated in FIG. 3, it is found that the forwardmovement of the aircraft produces sufficient airflow to remove thedroplets before they contaminate the electrodes. Too great an airflowwould produce air shear on the liquid on the surface 21, tending tostrip it off the surface before it reaches the edge 16. It is, however,possible to enhance the effect by the particular shape and position ofthe electrode aerofoils. A circumstance in which it might be desired toenhance the airflow is if the droplets are found otherwise to bedepositing on the electrodes. Such a condition might arise if it werenecessary to make the electrodes large in order to achieve stiffness.Suitable aerofoils to enhance the flow are illustrated in FIG. 4. Theseare substantially flat in section on the side remote from the edge 16,so encouraging the beneficial air flow through the space between themand the spray head, at the expense of the airflow around the outside. Inthis arrangement, the position of the electrode bodies encourages theairflow to be in the direction of the ligaments, with no substantialtransverse component, assisting the spray head to resist stalling.

Without the provision of the air flow between the electrodes and thespraying edge, if the conducting or semiconducting surface 20 werearranged at earth potential and the electrodes 4 were at a high(positive or negative) voltage, most of the droplets would deposit onthe electrodes. With the present provision of an air flow, it ispossible to spray using such an arrangement. A sufficient flow of nonturbulent air can protect the electrodes even in this extreme case.

The nose assembly 12 of the sprayhead comprises two parts: a skin 12aand a generally I-section beam 12b. Both are manufactured from glassreinforced plastics by pultrusion. The skin 12a and beam 12b areassembled together by screws leaving a hollow cavity 38 through whichpipework and high tension electrical leads (not shown in FIG. 3) supplyliquid to be sprayed and high voltage respectively, to the nozzle. Thenozzle assembly 14 conforms to the exterior shape of the nose assembly12 so as to form part of the aerofoil section. The nozzle assembly 14has a projection 40 along its length, which is a push fit between theflanges of the beam 12b. Push together electrical and fluid connectors(not shown) are provided between the beam 12b and the projection 40, sothat the nozzle assembly can be plugged into the nose assembly andeasily removed for service or replacement. The fluid connectorcommunicates with a distribution channel 44 in the interior surface ofthe nozzle part 14b. The distribution channel 44 conveys liquid to besprayed from the passage 40 to the slot 18.

As can be seen in FIG. 2, the sprayheads are not horizontal but conformto the dihedral of the aircraft's wings. During spraying the liquid issupplied under positive pressure from a metering pump (not shown) andthe dihedral causes no problem. However, when the aircraft reaches theend of its run over one strip, the spray is turned off while it turns tospray an adjacent strip. If there were one continuous slot 18 throughoutthe length of the sprayhead, there would be a tendency for the liquid torun towards the lower end of the slot leaving the upper end empty whenthe liquid. This would leave a short lag between the time the meteringpumps were switched on and the time spraying commenced, which wouldleave an indeterminate and unacceptable area unsprayed. This problem isovercome by dividing the slot 18 into short independent sections, eachsupplied with liquid separately and each short enough that capillaryaction is sufficient to keep the sections full from end to end at normalattitudes and in manoeuvers normal during spraying.

Referring to FIG. 5, the sprayhead is manufactured in standard lengthsections. Eight sections 14.1 to 14.8 of nozzle 14 are illustratedschematically. In each section there are three separate sections of slot18, the sections being isolated by separators provided in the spacerdefining the slot 18.

Each section of slot 18 is fed by a respective separate distributionpassage 44 from a respective separate fluid connector. Between the fluidconnector and the respective distribution gallery 44 is a non returnvalve 46 which prevents one section of distribution gallery 44 draininginto another. Each section of nozzle 14.1 to 14.8 has three isolatedsections of slot 18 and distribution gallery 44, these being fed from acommon duct 41 via a non return valve 48 and a flow regulator 42.

A problem is caused by the need to use non return valves to isolate theseparate sections of the distribution gallery 44 and slot 18, in thatthe sorts of solvents used for pesticides for electrostatic spraying,are highly damaging to most elastomeric materials. Non return valves notusing elastomers as a seal tend to rely on high spring pressures to keepthem shut. This in turn leads to the valve not opening at low forwardpressure and variations between valves of the flow rate at a particularpressure. This does not matter so much for the valves 48 as each isassociated with a flow regulator 42. However no such regulator isassociated with the valves 44. This problem can be overcome by the usein the non return valves 44, of a PTFE O-ring as a seal. SuitableO-rings are available under the trade name "CHEMRAZ" from Green Tweedand Co Inc, Detweiler Road, Kulpsville, USA.

As there is no direct connection possible, maintaining a voltagereference relative to ground poses something of a problem. A solution isdescribed in EP-A2-0186353. Applied to the present apparatus, thecircuit arrangement is shown in FIG. 3.

As shown in FIG. 6, the aircraft carries two spray head/electrodeassemblies 2a, 4a and 2b, 4b. These are mounted one on each said of theaircraft, as shown in FIG. 2. There are two high voltage generators 50,52, each powered by a battery 54. Each generator has two high voltageoutputs reference to a respective ground terminal 58, 60. Both of whichare connected to the body or airframe 61 of the aircraft. A -35 Kvoutput 62 of the generator 50 is connected to the surface 20a of thespray head 2a. A -17.5 Kv output 64 of the generator 50 is connected tothe associated electrodes 4a. Similarly, a +35 Kv output 66 is connectedto the surface 20b of the spray head 2b and a +17.5 Kv output isconnected to the associated electrodes 4b. The generators 50 and 52 arepreferably mounted in the nose assemblies of their respectivesprayheads. This removes the need to make high voltage electricalconnections to the sprayheads, only low voltage external connectionsbeing necessary.

It will be appreciated that atomised liquid emerging from the spray head2b is charged positively. Liquid emerging from the spray head 2a ischarged negatively. During spraying, positive current from the generator52 flows to ground via the terminal 66, the conducting or semiconductingsurface 20b in the spray head 2b and the liquid emerging from the sprayhead. In the absence of the connection between terminals 58 and 60,there would be no return lead for current to flow back to the generator52 from the ground (i.e. the target). Accordingly a negative chargewould build up on the generator 54.

This build up of charge on the generator 52 reduces the potential withrespect to the electrode 4b, which is applied to the conducting orsemiconducting surfaces 20b, thus reducing the atomising field and thecharge applied to the spraying liquid. There is therefore an increase inthe size of the droplets of liquid and a deterioration of sprayingquality. The generator 50 would be affected similarly.

In practice if one of the generators 50 or 52 supplies more current thanthe other, a charge builds up on the generators. The polarity of thecharge is such as to reduce the atomising field on the spray headsupplied by the generator supplying the greater current. This reducesthe quality of the spray from the associated spray head and the spraycurrent from the generator is also reduced.

Conversely, the atomising field on the spray head supplied by the othergenerator supplying the smaller current, is increased. The quality ofthe spray from this spray head is therefore improved and the spraycurrent increases until it matches that from the first generator.

In an alternative arrangement, the slot 18 can be at, rather than aheadof, the trailing edge 16. Although such an arrangement may appear tocreate two spraying edges because the slot, naturally, has two sides,the electrostatic effect is that of one edge. That is to say only oneset of ligaments is formed centrally. If the electrostatic effect werethat of two edges, ligaments would be produced off the "edges" at bothsides of the slot. This concept of one edge fed by a central slot may,perhaps be better understood by considering that the fluid to be sprayedhas significant conductivity and will, in use, bridge the slot.

In a yet further arrangement, more than one slot 18 can be arranged tofeed liquid to a single spraying edge.

We claim:
 1. Apparatus for spraying liquid electrostatically into anairstream comprising: a sprayhead having a spraying edge, anelectrically conducting or semiconducting surface and means fordelivering liquid to be sprayed to the edge via the surface; anelectrode spaced from the edge; and high voltage supply means forgenerating a high voltage between the surface and the electrode, thesprayhead comprising an aerofoil the trailing edge of which constitutesthe spraying edge, the sprayhead and the electrode being mounted forpart of the airstream to pass between them, the shape and position ofthe sprayhead and the electrode producing a sufficiently low turbulencewake in the region of the spraying edge, and the electric field at theedge being intensified sufficiently when covered by liquid to besprayed, that: the liquid at the edge is drawn out preponderantly byelectrostatic forces into ligaments which break up into electricallycharged droplets.
 2. Apparatus as claimed in claim 1, wherein thesprayhead is a symmetrical aerofoil.
 3. Apparatus as claimed in claim 2,including two electrodes, one on each side of the trailing/sprayingedge.
 4. Apparatus are claimed in claim 1, wherein the electrode is anaerofoil.
 5. Apparatus as claimed in claim 4, in which each electrodecomprises a conducting core and a sheath or cover at least partly formedof semi-insulating material having a resistivity in the range 5×10¹¹ to5×10¹³ ohm cm.
 6. Apparatus as claimed in claim 5, in which the core andthe sheath or cover are so shaped as to be closest to the sprayhead atthe spraying edge, at least the part of the sheath or cover nearest thespraying edge being formed of said semi-insulating material. 7.Apparatus as claimed in claim 6, wherein the core is packed iron filingsor carbon granules and the sheath or cover is said aerofoil. 8.Apparatus as claimed in any one of claims 1, 3, 4, and 7, wherein thespraying edge and the electrode or electrodes are linear.
 9. Apparatusas claimed in claim 1, including a series of separate slots closelyspaced along the length of the spraying edge for feeding liquid thereto,and a respective non return valve arranged to supply liquid to each slotvia a respective distribution gallery.
 10. Apparatus as claimed in claim9, including a plurality of further non return valves, each arranged tofeed liquid to a group of adjacent first mentioned non return valves.11. Apparatus as claimed in claim 10, including a common feed passage todistribute liquid via individual passages to each of the said furthernon return valves, and a flow restricting plug extending into eachindividual passage from common feed passage by an amount to regulate theflow through each respective further non return valve.